1. Field of the Invention
The present invention relates to a battery charger for charging a secondary battery, such as a nickel cadmium battery, which battery charger is capable of rapidly and reliably charging a variety of batteries with varying cell numbers.
2. Description of the Prior Art
U.S. Pat. No. 4,998,057 describes a technology for rapidly and reliably charging a variety of secondary batteries with varying cell numbers. This technology includes a plurality of voltage division resistors having different resistances. The voltage division resistors are used to divide the voltage across the battery. A relevant voltage division resistor is selected by a microcomputer depending on the battery voltage so that an input voltage applied to an A/D converter may fall within a predetermined range regardless of the number of cells, that is, regardless of the battery voltage.
With this technology, the fully charged condition of the battery can be accurately detected by a so-called -.DELTA.V detection method in which charging of the battery is stopped when a predetermined voltage drop (-.DELTA.V) is detected after the voltage of the battery in the charging process reaches the peak. This technology can also be used in conjunction with other methods for detecting the fully charged condition of the battery, such as a method for stopping charging before the battery voltage reaches the peak or a second order differential detection method. The former method is advantageous in preventing the battery from being overcharged and improving the recharging cycle life of the battery, that is, the number of times the battery can be charged. The latter method uses a second order differential of the battery voltage during charging process differentiated by time. When the second order differential of the battery voltage becomes negative, it is determined that the battery voltage has reached the peak.
Among others, the second order differential detection method, which allows fewer overcharges than the -.DELTA.V detection method, is gradually becoming the main detection method in use. The second order differential detection method increases the cycle life of batteries by allowing fewer overcharges than the -.DELTA.V detection method, decreasing the number of times pressure increases occur in the battery as a result of oxygen gas being generated in the final stages of charging. Further, leakage of electrolytic solution in the battery is decreased through the operation of a safety valve included in the battery. The second order differential detection method is particularly widely used for charging batteries in which charging and discharging is performed with a large current, as is with the batteries in electric tools.
However, when charging inactive batteries, such as batteries that are new or have been left unused for a long period of time, the amount of voltage change is small, and such batteries are, therefore, very difficult to determine the fully charged condition using the second order differential detection method.
FIG. 3 is a graphical representation showing a battery voltage during the charging process and also a digital value corresponding to the voltage increase. The battery voltage is sampled at a predetermined time interval, and the voltage sampled is converted by an 8-bit A/D converter to a digital value ranging from 0 to 255 in decimal notation. The new digital value of the battery voltage is compared with a previous digital value and the increase in the digital value corresponding to the voltage increase is shown in the graph of FIG. 3.
As seen in this graph, the rise in voltage over time when charging an inactive battery is gentle in comparison to that of an active battery, shown in FIG. 5. The voltage in the final stage of charging the inactive battery peaks slowly, and since the amount of voltage change is small, it is not possible to reliably detect that the battery has reached the fully charged condition using the second order differential detection method to determine when the second order differential of the battery voltage becomes negative. As shown in FIG. 3, the digital value representing the voltage increase of the inactive battery is only 1. This example uses a 10-cell battery. The voltage developed across the battery is subjected to a voltage division with a ratio of 0.203. Therefore, the voltage increase corresponding to one digital value would be 5/0.203.times.1/255 =96.6 mV, or 9.66 mV/cell where the reference voltage of the 8-bit A/D converter is 5 volts. In this example, the second order differential value for the battery voltage is determined negative when the voltage drops from 1 to 0 in digital value. Since such change in the digital values occurs frequently from the beginning of the charging process, a fully charged condition of the battery cannot be reliably detected. Hence, in order to reliably detect a full charge by determining when the second order differential value of the battery voltage becomes negative, the converted voltage increase must be at least 2 in digital value. The charging process would then be stopped when the converted voltage drops at least 2 in digital values. In this case, a full charge cannot be reliably determined when charging an inactive battery.
FIG. 4 shows the charging of a battery that is warm because it is just after discharge. The voltage of this battery in the final stage also peaks slowly. For the same reasons as described above, a charging control method that detects a negative change in voltage cannot reliably detect the amount of voltage change with the limited resolution of an A/D converter.
In recent years, the demand to replace nickel cadmium batteries with nickel hydrogen batteries has grown in response to a need for higher capacity. Still, nickel hydrogen batteries have the same problems as described above, because the peak at full charge is not as sharp as nickel cadmium batteries.
Further, for a universal battery charger designed to charge batteries with varying cell numbers, for example all batteries with an even number of cells from a 4-cell battery to a 20-cell battery, a total of nine voltage division ratio settings are required. Naturally, in order to select one of the voltage division resistors to suit a battery voltage using a microcomputer, the microcomputer must be equipped with nine output ports, increasing the number of pins in the microcomputer and, as a result, increasing the size of the microcomputer.